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They're what's among us.

Fungus (plural, fungi) is one of the kingdoms of life. The fungi, although traditionally included with plants (as Richard Dawkins puts it, they "are rooted to the spot and don't walk away while you try to study them"[1]), turned out to be more closely related to the animals. Fungi, having been around for over 500 million years, are extremely diverse and widespread, including molds (except slime molds), mushrooms, toadstools, yeasts, and many other organisms.

Fungi have many uses. In the environment, fungi feature as they decompose dead matter, by growing root-like hyphae (which form a dense mycelium), digesting their food on the outside, and absorbing it. In food, fungi not only top pizzas but work in blue cheese, beer, bread, soy sauce, and many other products. More importantly, however, fungi were the source of the first antibiotics, including penicillin, a mold product. Some fungi are toxic, including some mushrooms. Some people take "advantage" of this with "magic mushrooms", a hallucinogen. Worse, though, some fungi are even parasites on humans, causing diseases like athlete's foot (itchy foot) or vaginitis (itchy stuff).

Fungi do not, however, cause cancer, despite what some people say. A certain fungus that can live in your gut, named Candida, is basically harmless but has been blamed for nearly all human ills by certain members of the alternative medicine community.

Fungi can be seen as the creationist's nightmare, much like how the banana or peanut butter is the atheist's, only this one is actually good. In Genesis 1, God is mentioned as creating plants and animals, with no mention at all of the world's other life forms. Also, fungi feed on dead remains, so before original sin, fungi would have had nothing to eat. The only way around this really is for the creationist to claim that fungi appeared after the Fall, perhaps in some sort of super-evolution or something. Given how this would be one kingdom arising from another (plants or animals? take your pick), this seems to be life evolving "between kinds".